The realization of sound generating structures based on micro fabrication, or micro electro mechanical systems (MEMS), technology is particularly desirable as the utilization of the high-volume batch fabrication technology may reduce the device size, and improve the device quality, yield, and performance-to-cost ratio of such devices. The fundamental problem with sound generation, in contrast to sound detection, is that the device must provide a certain air volume displacement to generate a certain sound pressure. If the area of the sound generating structure (i.e. diaphragm) is reduced, to reduce the overall device size, the result is that the structure must have a larger displacement to generate the same sound pressure. A consequence of this is that the force necessary to drive the diaphragm increases. This is not easily combined with the reduction of the actuator size, since smaller actuators in general provide less actuation force. This scaling issue has proven prohibitive for micro scale implementations of established electromagnetic actuation principles, which are common in larger conventional acoustic transducers, since the actuation force needed is beyond the reasonable capability of electromagnets with excessive power consumption as a result.
There are transduction principles that can generate the necessary forces on the micro scale. The problem is that the force must be generated over a relatively large physical travel of the actuator. This generally disqualifies all piezoelectric actuators, since such devices can generate large strains and forces, but with very limited travel. A more promising actuator technology is based on electrostatic attraction forces that are caused by opposing electrical charges built up on conductive surfaces. Since the electrostatic force is inversely proportional to the square of the distance between the conductors, potentially very large forces can be generated if the conductors are in close proximity. In particular, if an actuator is used in which the conductors come into physical contact, only being separated by a solid insulator, the electrostatic force can be increased substantially if the solid insulator has a high relative permittivity and is very thin. An electrostatic transducer based on an electrostatic actuator principle has been disclosed in U.S. Pat. No. 6,552,469 and is shown in cross-section in FIG. 1. This prior art structure involves a micro fabricated cantilever actuator, which is attached to an external membrane with a support brace. The fabrication of such a support brace and membrane would be cumbersome in high-volume manufacturing, and it would be desirable to integrate all structural components to realize a smaller structure.